Is there a bug on SO or something wrong with my account?
I've hit the daily rep cap and I've got upvotes (during the same day) on some of my accepted answers without a corresponding increase in rep.
It isn't the first time this happens.
Well, my rep tracker certainly reckons you've hit the rep cap.
When you say "without a corresponding increase in rep" are you suggesting you were on exactly 12173 rep at the start of the UTC day?
If you're wondering why you haven't broken through the rep cap: it's not votes on accepted answers that are immune to the limit - it's the 15 point bonus for having an answer accepted. It's not entirely clear from your question, but it's possible that you've got slightly the wrong end of the stick about the rep cap.